Essentially, the heart is a pump which pumps blood throughout the body. It consists of four chambers--two atria and two ventricles. In order for the heart to efficiently perform its function as a pump, the atrial muscles and ventricular muscles should contract in a proper sequence and in a timed relationship.
In a given cardiac cycle (corresponding to one "beat" of the heart), the two atria contract, forcing the blood therein into the ventricles. A short time later, the two ventricles contract, forcing the blood therein to the lungs (from the right ventricle) or through the body (from the left ventricle). Meanwhile, blood from the body fills the right atrium and blood from the lungs fills the left atrium, waiting for the next cycle to begin. A typical healthy adult heart may beat at a rate of 60-80 beats per minute (bpm) while at rest, and may increase its rate to 140-180 bpm when the adult is engaging in strenuous physical exercise, or undergoing other physiologic stress.
The healthy heart controls its rhythm from its SA node, located in the upper portion of the right atrium. The SA node generates an electrical impulse at a rate commonly referred to as the "sinus" or "intrinsic" rate. This impulse is delivered to the atrial tissue when the atria are to contract and, after a suitable delay (on the order of 140-220 milliseconds), propagates to the ventricular tissue when the ventricles are to contract.
When the atria contract, a detectable electrical signal referred to as a P-wave is generated. When the ventricles contract, a detectable electrical signal referred to as an R-wave is generated. The R-wave is much larger than the P-wave, principally because the ventricular muscle tissue is much more massive than the atrial muscle tissue. The atrial muscle tissue need only produce a contraction sufficient to move the blood a very short distance--from the respective atrium to its corresponding ventricle. The ventricular muscle tissue, on the other hand, must produce a contraction sufficient to push the blood over a long distance (e.g., through the complete circulatory system of the entire body).
Other electrical signals or waves are also detectable within a cardiac cycle, such as a Q-wave (which is the negative deflection immediately preceding an R-wave), an S-wave (which is the negative deflection immediately following an R-wave), and a T-wave (which represents the repolarization of the ventricular muscle tissue).
It is the function of a pacemaker to provide electrical stimulation pulses to the appropriate chamber(s) of the heart (atrium, ventricle, or both) in the event the heart is unable to beat on its own (i.e., in the event either the SA node fails to generate its own natural stimulation pulses at an appropriate sinus rate, or in the event such natural stimulation pulses do not effectively propagate to the appropriate cardiac tissue). Most modern pacemakers accomplish this function by operating in a "demand" mode where stimulation pulses from the pacemaker are provided to the heart only when it is not beating on its own, as sensed by monitoring the appropriate chamber of the heart for the occurrence of a P-wave or an R-wave. If a P-wave or an R-wave is not sensed within a prescribed period of time (which period of time is often referred to as the "escape interval"), then a stimulation pulse is generated at the conclusion of this prescribed period of time and delivered to the appropriate heart chamber via a pacemaker lead.
Modern programmable pacemakers are generally of two types: (1) single-chamber pacemakers, and (2) dual-chamber pacemakers. In a single-chamber pacemaker, the pacemaker provides stimulation pulses to, and senses cardiac activity within, a single-chamber of the heart (e.g., either the right ventricle or the right atrium). In a dual-chamber pacemaker, the pacemaker provides stimulation pulses to, and senses cardiac activity within, two chambers of the heart (e.g., both the right atrium and the right ventricle). The left atrium and left ventricle can also be paced, provided that suitable electrical contacts are made therewith.
Much has been written and described about the various types of pacemakers and the advantages and disadvantages of each. For example, commonly-assigned U.S. Pat. No. 4,712,555 of Thornander et al. presents background information about pacemakers and the manner in which they interface with a patient's heart. This patent is hereby incorporated by reference in its entirety.
One of the most versatile programmable pacemakers available today is the DDDR pacemaker. This pacemaker represents a fully automatic pacemaker which is capable of sensing and pacing in both the atrium and the ventricle, and is also capable of adjusting the pacing rate based on one or more physiological factors, such as the patient's activity level. When functioning properly, the DDDR pacemaker can limit certain drawbacks associated with the use of pacemakers. For example, the DDDR pacemaker can maintain AV synchrony while providing bradycardia support.
In general, DDDR pacing has four functional states: (1) P-wave sensing, ventricular pacing (PV); (2) atrial pacing, ventricular pacing (AV); (3) P-wave sensing, R-wave sensing (PR); and (4) atrial pacing, R-wave sensing (AR). Advantageously, for the patient with complete or partial heart block, the PV state of the DDDR pacemaker tracks the atrial rate which is set by the heart's SA node, and then paces in the ventricle at a rate that follows this atrial rate. Because the rate set by the SA node represents the rate at which the heart should beat in order to meet the physiologic demands of the body (at least for a heart having a properly functioning SA node) the rate maintained in the ventricle by such a pacemaker is truly physiologic.
Those skilled in the art have long recognized the advantages of using an atrial tracking pacemaker. For example, U.S. Pat. No. 4,624,260 to Baker, Jr. et al. discloses a microprocessor-controlled dual-chamber pacemaker having conditional atrial tracking capability. Similarly, U.S. Pat. No. 4,485,818 of Leckrone et al. discloses a microprocessor-based pacemaker which may be programmed to operate in one of a plurality of possible operating modes, including an atrial rate tracking mode.
Unfortunately, in some instances, a given patient may develop fast atrial rhythms which result from a pathologic arrhythmia such as a pathological tachycardia, fibrillation or flutter. In these cases, a DDDR pacemaker may pace the ventricle in response to the sensed atrial arrhythmia up to a programmed maximum tracking rate (MTR). The MTR defines the upper limit for the ventricular rate when the pacemaker is tracking the intrinsic atrial rate. As a result, the MTR sets the limit above which the ventricles cannot be paced, regardless of the intrinsic atrial rate. Thus, the purpose of the MTR is to prevent rapid ventricular stimulation, which could occur if the intrinsic atrial rate becomes very high and the pacemaker attempts to track atrial activity with 1:1 AV synchrony.
When the intrinsic atrial rate exceeds the MTR the pacemaker may initiate one or more upper atrial rate response functions--such as automatically switching the pacemaker's mode of operation from an atrial tracking mode to a non-atrial rate tracking mode. However, in some cases mode-switching may not be a desirable upper rate response. Most previously known mode-switching techniques are based in whole or in part on the patient's sensed atrial rate exceeding a certain threshold atrial rate (such as the MTR). This mode-switching criterion may cause problems for patients who exhibit normal sinus tachycardia due to physical activity. Another difficulty associated with mode-switching techniques is that mode-switching occasionally occurs due to electrical noise present in the atrial sensing channel of the pacemaker, or due to a one-of-a-kind fast P-wave. In these instances, rates slightly exceeding the MTR are not indicative of a pathologic arrhythmia. These patients may thus be subjected to undesirably frequent mode-switching occurrences as their atrial rates exceed and then drop below the MTR.
The heart's natural response to a high atrial rate involves a phenomenon known as "blocking"--where the AV node attempts to maintain a form of AV synchrony by "dropping out" occasional ventricular beats when the high atrial rate exceeds a certain natural threshold i.e., the refractory period of the heart tissue. The blocking phenomenon is often expressed as a ratio of the atrial beats to the ventricular beats (e.g. 6:5, 4:3, etc.). Of particular importance is a 2:1 block condition where there are two atrial beats for every one ventricular beat. The 2:1 block condition is a natural response to a very high atrial rate, during which full ventricular rate synchronization (i.e. at a 1:1 ratio) would be dangerous to the patient.
Implantable stimulation devices emulate this 2:1 condition, by tracking P-waves up to the device's programmed total refractory period (TARP) of the heart. That is, P-waves which fall in the total refractory period are not tracked, and the device is said to have a "2:1 response mode".
In addition, a 2:1 block response mode decreases the likelihood of a pacemaker mediated tachycardia. Pacemaker mediated tachycardia may occur when pacing pulses delivered in the ventricle causes a retrograde P-wave to be conducted to the atria immediately after each pacing pulse is delivered. This forces the apparent atrial rate to increase due to the additional P-waves occurring during a cardiac cycle during which a ventricular pulse was delivered. Since the ventricle is typically paced in full synchrony with the atrial rate, a tachycardia develops as the ventricular pacing rate increases to follow the high atrial rate caused by the retrograde conduction. During the 2:1 block response mode, the ventricles are paced at a lower rate than the atrial rate, because P-waves occurring soon after ventricular events are ignored for the purposes of calculating the ventricular pacing rate. As a result, the 2:1 block response mode prevents the pacemaker from pacing the ventricles at a tachycardia rate.
The 2:1 block response mode is an effective response for dealing with short incidences of high atrial rates and in preventing occurrence of a pacemaker mediated tachycardia resulting from retrograde P-waves. However, the 2:1 block response mode may become uncomfortable for the patient if it is maintained for an extended period of time due to programmed long atrial refractory periods, because the pacing rate will be 1/2 the required physiologic rate.
Some dual-chamber pacemakers have attempted to emulate the natural block condition as an upper atrial rate response by reducing the ventricular pacing rate when the intrinsic atrial rate exceeds the MTR. Since the MTR is programmed by the medical practitioner as the maximum safe rate at which the ventricles may be paced, when the intrinsic atrial rate exceeds the MTR a dangerous upper rate condition is deemed to exist and the ventricular pacing rate is reduced in order to prevent it from exceeding the MTR.
Pacemakers incorporate a programmable parameter known as an atrial refractory period, which is initiated by either a paced or sensed cardiac event. The atrial refractory period, also called a total atrial refractory period (TARP), is made up of two segments. The first segment, known as the AV Delay (AVD), is initiated by a paced or sensed atrial event. The second segment, known as the post ventricular atrial refractory period (PVARP), is initiated by a paced or sensed ventricular event. The TARP (measured in milliseconds (ms)) is inversely proportional to the rate at which 2:1 block occurs, that is, the 2:1 block rate occurs is 60,000.div.TARP for conversion to beats per minute (bpm). As a result, when the intrinsic atrial rate exceeds the rate specified by the TARP, one or more atrial events occur during the TARP. For example, if the 2:1 block rate is 160 bpm, then the TARP is 60,000.div.160, or 375 ms. If the intrinsic atrial rate is 200 bpm, the interval between the atrial events is 60,000.div.200, or 300 ms, which means that one atrial event occurs within the TARP of 375 ms. Therefore, atrial events occurring during the TARP are not counted by the pacemaker for the purpose of pacing the ventricles. Instead, only atrial events occurring outside the TARP are counted in order to derive a "sensed functional atrial rate". The pacemaker then paces the ventricles at a rate that follows the sensed functional atrial rate. As the intrinsic atrial rate increases, the occurrence of an atrial event during the TARP becomes more probable, until every other atrial event falls into the TARP. Since the atrial events occurring during the TARP are not counted, the sensed functional atrial rate (and thus the ventricular pacing rate) is approximately one half of the intrinsic atrial rate and a 2:1 block response mode is entered. When the intrinsic atrial rate begins to decrease, a decreasing number of atrial events occur within the TARP. When the intrinsic atrial rate falls below the 2:1 block rate, it is equal to the sensed functional atrial rate, since no atrial events occur during the TARP (i.e., no atrial events are skipped).
Advancements in pacemaker technology have been driven by a desire to approximate true physiological cardiac activity through pacing. One pacemaker function that mimics physiological behavior of the heart is rate-responsive AV delay (RRAVD). The RRAVD allows the pacemaker to respond to changes in the intrinsic atrial rate by progressively decreasing the AVD in preprogrammed increments from its base value as the intrinsic atrial rate increases until a minimum preset shortened AVD value is reached. This function is referred to as "rate-responsive AV delay shortening". This minimum preset AVD is usually reached when the intrinsic atrial rate exceeds an upper rate threshold but before the 2:1 block condition is entered. Thus, the RRAVD combined with the PVARP enables the TARP to change in response to changes in the atrial rate. A rate-responsive refractory period is advantageous because it closely emulates the physiological behavior of the heart and increases patient comfort.
Similar to most rate-responsive pacemaker functions, the RRAVD of previously known pacemakers is based on the sensed functional atrial rate since previously known rate-responsive pacemakers equipped with the RRAVD function do not sense atrial events occurring during the TARP. This does not pose a problem when the intrinsic atrial rate is below the 2:1 block rate since up to that point, the intrinsic atrial rate is equal to the sensed functional atrial rate. However, when the intrinsic atrial rate exceeds the 2:1 block rate, the sensed functional atrial rate begins to drop in value as atrial events falling into the TARP are ignored. When the 2:1 block response mode is reached at a block entry rate, which is typically equal to the TARP, the RRAVD is reset from its minimum value to its base value, because according to the sensed functional atrial rate, the atrial rate is far below the upper rate limit. This phenomenon results in an adjustment of the TARP to a higher value and thus changes a block exit rate at which the 2:1 block condition may be exited. As a result, the block exit rate is lower than the block entry rate, because the block exit rate is based on the higher TARP (incorporating the maximum value AVD), while the block entry rate is based on a lower TARP (incorporating the minimum AVD). Thus, a patient will be forced to remain in the 2:1 block response mode longer than necessary, because in order for the pacemaker to exit from the 2:1 block response mode, the intrinsic atrial rate must drop below the block exit rate, a lower rate than the block entry rate at which the 2:1 block response mode was entered.
Inconsistent rates of entry into and exit from 2:1 block are contradictory to the physiological behavior of the heart (where the rates of entry into and exit from naturally occurring 2:1 block condition are relatively consistent), and may cause discomfort to the patient. Furthermore, inconsistent rates of entry into and exit from 2:1 block force the pacemaker to maintain the 2:1 block response mode longer than is necessary further increasing the likelihood of discomfort associated with an extended 2:1 block response mode.
Thus, it would be desirable for a pacemaker equipped with a RRAVD function to maintain consistent rates of entry into and exit from a 2:1 block response mode.